The present invention relates to single-chain thrombomodulin polypeptides, including analogs of thrombomodulin ("TM") that are less susceptible to cleavage by proteases. These analogs are useful in, for example, antithrombotic therapy. Novel proteins, nucleic acid gene sequences, pharmaceuticals, and methods of inhibiting thrombotic activity are disclosed.
There are many disease states that would benefit from treatment with a safe and effective anticoagulant/anti-thrombotic. The nature of these conditions varies. For example, anticoagulant therapy is useful in acute conditions such as during thrombolytic therapy in myocardial infarction or in treatment of disseminated intravascular coagulation (DIC) associated with, for example, septicemia. Anticoagulants are also useful for less acute conditions, such as chronic use in patients that have received heart valve implants or prophylactic use in surgery patients to reduce the risk of deep venous thrombosis (DVT).
Thrombomodulin is a membrane protein that has demonstrated anticoagulant properties. Its physiological importance has been studied. (See, for example, N. Esmon, et al., (1982) J. Biol. Chem. 257:859-864, H. Salem, et al., (1983) J. Biol. Chem. 259:12246-12251).
The gene encoding native thrombomodulin has been isolated and sequenced from several species, both in its genomic form and as a CDNA clone (Jackman, R., et al., (1986) Proc. Natl. Acad. Sci. USA. 83:8834-8838 and (1987) 84:6425-6429, both of which are herein incorporated by reference). Comparisons with known proteins, such as the LDL receptor, have suggested functional domains (Wen, D., et al., (1987) Biochemistry 26:4350-4357). One study has suggested that the fifth and sixth epidermal growth factor (EGF)-like domains have the capacity to bind thrombin (Kurosawa, S., et al., (1988) J. Biol. Chem. 263:5993-5996; another suggests that EGF-like domains 4, 5, and 6 are sufficient to act as a cofactor for thrombin-mediated protein C activating activity. (Zushi, et al., (1989) J. Biol. Chem. 264:10351-10353). Inhibition of thrombin's direct procoagulant activity (conversion of fibrinogen to fibrin) has been attributed in part to glycosaminoglycan substituents on the thrombomodulin molecule. (Bourin, M. C. et al., (1986) Pro. Natl. Acad. Sci. USA 83:5924-5928.) The O-linked glycosylation domain has potential sites for the addition of these types of sulfated sugars.
Thrombomodulin analogs, including soluble molecules, having various modifications are known. There are, for example, modifications to oxidation-sensitive amino acid residues in thrombomodulin which render the molecule resistant to oxidation. There are also modifications to thrombomodulin, e.g., by elimination of sulfated o-linked carbohydrates through enzymatic removal or modification of glycosylation sites on the peptide, which decrease the inhibition of thrombin-mediated platelet aggregation and thrombin-mediated conversion of fibrinogen to fibrin, which is an important property of thrombin. These modifications are disclosed in U.S. Ser. No. 07/568,456, filed Aug. 15, 1990, which is incorporated herein by reference.
Anticoagulants currently approved for use in humans are not uniformly effective and a need exists for more efficacious compounds (See, for example, Prevention of Venous Thrombosis and Pulmonary Embolism, Consensus Development Conference Statement, NIH, 1986, 6(2):1-23).
Thrombomodulin in its native form is not suitable for anticoagulant therapy as it is membrane-bound, due to its inherent amino acid sequence, and is insoluble without detergent treatment. It is present in such small amounts (about 300 mg thrombomodulin/person) that purification from autopsy or biopsy samples is impractical.
Soluble thrombomodulin-like molecules have been detected at very low amounts in human plasma and urine. These molecules have a reduced ability to promote protein C activation, and it is possible that they have been rendered at least partially inactive, due at least in part, to oxidation. It has been suggested that these molecules are degradation products of the membrane bound molecule (Ishii, H. and Majerus, P., (1985) J. Clin. Inv. 76:2178-2181), but they are present in such low amounts that they have been difficult to characterize (.about.0.8 mg/adult male). Proteolytic fragments of the purified native molecule have been produced using trypsin or elastase. (See, Ishii, supra, Kurosawa, et al., (1988) J. Biol. Chem. 263:5593-5996 and Stearns, et al., (1989) J. Biol. Chem. 264:3352-3356). Some of these fragments retain the ability to promote thrombin-mediated activation of protein C in vitro.
The production of TM and TM analogs by recombinant techniques in heterologous cells, e.g., in cell culture, has encountered numerous problems in achieving acceptable products for use in in vivo applications. For example, the N-terminal end of TM is imprecisely cleaved in cells containing the recombinant gene as well as in native cells, resulting in a product that has a non-unique N-terminus. This causes, among other difficulties, a problem in providing proof of purity of the isolated polypeptide, e.g., for regulatory purposes. Glycosylation of recombinantly produced TM has also proved to be a problem, in that some of the glycosylation sites are apparently involved in maximizing biological activity, while other sites are apparently recognized as signals in vivo to clear the bloodstream of TM, thus reducing the circulating half-life of a TM so glycosylated. Other, less well defined problems also are known to exist which interfere with the production of a maximally useful recombinantly produced TM polypeptide.
Thus, there is a need for new compositions that exhibit the anticoagulant properties of thrombomodulin and are easily produced in large quantities, but without the problems encountered in recombinant production of TM by heterologous cells. The present invention fulfills these and other needs.